Cylindrical β-sheet peptide assemblies

Recent reports have shown that cyclic peptides composed of an even number of alternating D- and L-amino acids can adopt flat, disklike conformations and stack through backbone-backbone hydrogen-bonding to form extended nanotubular structures. The present work details a general strategy for limiting this self-assembly process through backbone alkylation, giving rise to cylindrical beta-sheet peptide dimers. Scope and limitations of dimerization are examined through NMR, FT-IR, mass spectral, and X-ray crystallographic studies of 20 cyclic peptides varying in ring size, location and identity of backbone alkyl substituents, and amino acid composition. The cyclic peptides are shown to self-assemble both in solution and in the solid state through the expected antiparallel beta-sheet hydrogen-bonding network. While solution dimerization by cyclic octapeptides appears general, peptides with alternative smaller or larger ring sizes fail to self-associate. Formation of cylindrical beta-sheet ensembles is found to tolerate a number of backbone N-alkyl substituents, including methyl, allyl, n-propyl, and pent-4-en-1-yl groups, as well as a range of amino acid side chains. Within the hemi-N-methylated octapeptide framework, residues exhibit differential propensities for dimer stabilization, analogous to amino acid beta-sheet propensities in natural systems. Dimer-forming cyclic D,L-peptides are thus among the most structurally well characterized and synthetically accessible beta-sheet peptide model systems.